a. Field of the Invention
This invention pertains to devices for frequency multiplexing multiple signals upon one radio frequency carrier.
More particularly, this invention pertains to devices for generating a composite baseband signal that is used to modulate the radio frequency carrier. For instance, in stereophonic broadcasting systems, the radio frequency carrier typically is frequency modulated by a composite baseband signal, which composite baseband signal includes a main channel signal, a subchannel signal and, in many instances, a pilot subcarrier.
In the typical system the stereo input signal is composed of a "left" audio signal and a "right" audio signal. The composite baseband signal is the sum of the left and the right audio signals and a subchannel signal which consists of the sidebands that would be generated by a suppressed subcarrier, balanced modulator that is modulated by the difference between the left and right audio signals. The composite baseband signal may also include a pilot subcarrier that typically is one-half the frequency of the subcarrier. The pilot carrier is used as a reference for the frequency and phase of the subcarrier for use in connection with the demodulation of the subcarrier sidebands.
The Engineering Handbook, National Association of Broadcasters, Department of Technology, Washington D.C. (referred to herein as the "NAB Handbook") describes in some detail the technical specifications for such stereophonic frequency modulated signals and for the composite baseband signal.
If the left audio signal is represented by L, where L is a time varying function, and the right audio signal is similarly by R, and the subcarrier is represented by sin(wt) the composite baseband signal is represented by the sum of the left and right audio signals and the product of the subcarrier by the difference of the left and right audio signals, that is: ##EQU1## The pilot subcarrier having a frequency of one-half of the frequency of the subcarrier may or may not be present.
The present invention is a device for generating the composite baseband signal. Although a primary application of the present invention is the generation of composite baseband signals for use in stereophonic FM broadcasting systems, the invention is also applicable to other communication systems, including but not limited to television stereophonic transmission systems, that utilize subchannels for the transmission of multiple signals over one communication system. A pilot subcarrier may be included the invention, but it is not a necessary element of the present invention.
b. Description of the Prior Art
Chapter 4 of the NAB Handbook describes in conceptual terms various methods for generating the composite baseband signal. For instance, FIG. 1 hereof (FIG. 3 in Chapter 4 of NAB Handbook) depicts a frequency division multiplex stereo generator in which the left and right audio signals are combined in a matrix to produce the sum and difference of these signals. The difference between the left and right audio signals is applied to a balanced modulator operating at the subcarrier frequency and the output of the modulator is combined with the sum of the left and right audio signals to produce the composite baseband signal.
FIG. 2 hereof (FIG. 5 of chapter 4 of the NAB Handbook), by means of a block diagram, conceptually depicts a time division multiplex stereo generator which makes use of a switching modulator operating at the subcarrier frequency to generate the composite baseband signal.
FIG. 3 hereof (FIG. 6 of chapter 4 of the NAB Handbook), by means of a block diagram, depicts a time division multiplex stereo generator that utilizes a variable attenuator. Although the NAB Handbook describes these multiplex generators in conceptual terms, no mechanisms are described for effecting these various devices.
A company named C. N. Rood in the Netherlands markets a number of different stereo encoders. Its stereo encoder model SC-200 is apparently based on the concept of the variable attenuator depicted in FIG. 3.